Modification of 2D Silica Bilayer Structure via Strain and Al Doping

The preparation of 2D silicate bilayers on metal substrates in crystalline and amorphous forms paves the way to creating a multitude of 2D structures. We study strain effects on the bilayer structure using Ni$_{x}$Pd$_{1-x}$(111) films which allow continuous strain tuning from 6.0{\%} compressive to 3.8{\%} tensile. The interaction with the substrate and its atomic spacing and geometry determines the lattice strain exerted. Previously, a commensurate hexagonal crystalline bilayer was observed on Ru(0001) with a tensile mismatch of 2.2{\%}, while only amorphous bilayers were seen on Pt(111) (4.7{\%} tensile mismatch) and uniaxial strain on Pd(100) led to a commensurate crystalline phase with arrays of domain boundaries. Theory indicates that biaxial tensile strain above 2.5{\%} favors the introduction of 8-membered rings into the bilayer. Experiments show that SiO$_{2}$ bilayers grow in an incommensurate crystalline form on Pd(111) (x$=$0) which sets the maximum biaxial tensile strain to \textless 3.8{\%}. Meanwhile, AlSi$_{3}$O$_{8}$ thin films formed a commensurate crystalline film on Pd(111). Theory shows that the longer Al-O bonds reduce the strain energy by decreasing the mismatch to 1.9{\%}, explaining the transition from incommensurate to commensurate. Ongoing work on Ni$_{x}$Pd$_{1-x}$(111) is determining the maximum lattice strain that can be applied for controllable introduction of 8-membered rings into the structure.